1. Field
The field of the invention relates generally to Radio Frequency Identification (RFID) systems and more particularly to systems and methods for reading and writing information from multiple RFID enabled documents.
2. Background
FIG. 1 illustrates a basic RFID system 100. A basic RFID system 100 comprises three main components: an antenna or coil 104; an interrogator 102, and a transponder, or RF tag 106 which is often electronically programmed with unique information. Antenna 104 can be configured to emit radio signals 108 to activate tag 106 and read and write data from the activated tag 106. Antenna 104 is the conduit between tag 106 and interrogator 102, which is typically configured to control data acquisition and communication. Antennas 104 are available in a variety of shapes and size. For example, in certain embodiments they can be built into a door frame to receive tag data from persons or things passing through the door. In other embodiments, antennas 104 can, for example, be mounted on an interstate toll booth to monitor traffic passing by on a freeway. Further, depending on the embodiments, the electromagnetic field, i.e., radio signal 108, produced by an antenna 104 can be constantly present when, e.g., multiple tags 106 are expected continually. If constant interrogation is not required, then radio signal 108 can, for example, be activated by a sensor device.
Often antenna 104 is packaged with interrogator 102. A conventional interrogator 102 can emit radio signals 108 in ranges of anywhere from 1 inch to 100 feet or more, depending upon the power output and the radio frequency used. When an RFID tag 106 passes through an electromagnetic zone associated with radio signal 108, it detects radio signal 108, which can comprise an activation signal. In some embodiments, interrogators can comprise multiple antenna, though typically only one transmits at a time.
Additionally, interrogator 102 is often coupled through network 110 to a central server 112. Central server 112 can be configured to execute a number of applications including those that incorporate data from RFID tags 106. For example, in a tracking system, interrogator 102 transmits to the central server 112 the identity of tags that have passed through its interrogation zone. This information can be correlated to objects associated with the tag in a database residing on the central server and hence the whereabouts of the object in question at that particular time can be logged. In the example of a toll booth, tags that pass through the specific toll both are reported to central server 112, which correlates the tag to a motorist who is then debited the cost of the toll.
RFID tags 106 come in a wide variety of shapes and sizes. Animal tracking tags inserted beneath the skin, for example, can be as small as a pencil lead in diameter and one-half inch in length. Tags 106 can be screw-shaped to be inserted into trees or wooden items for identification purposes, or credit-card shaped for use in access applications. Anti-theft hard plastic tags attached to merchandise in stores can include RFID tags. In addition, heavy-duty RFID tags can be used to track containers, heavy machinery, trucks, and/or railroad cars for maintenance and/or tracking purposes.
RFID tags 106 are categorized as either active or passive. Active RFID tags 106 are powered by an internal battery and are typically read-write, i.e., tag data can be rewritten and/or modified. An active tag's memory size varies according to application requirements. For example, some systems operate with up to 1 MB of memory. The battery-supplied power of an active tag 106 generally gives it a longer read range. The trade off is greater size, greater cost, and a limited operational life.
Passive RFID tags 106 operate without a separate external power source and obtain operating power from radio signal 108. Passive tags 106 are consequently much lighter than active tags 106, less expensive, and offer a virtually unlimited operational lifetime. The trade off is that they have shorter read ranges than active tags 106 and require a higher-powered interrogator 102. Read-only tags 106 are typically passive and are programmed with a unique set of data, usually 32 to 128 bits, that cannot be modified. Read-only tags 106 often operate in the same way as linear barcodes. It should be noted that passive tags can also be used in read-write systems.
RFID systems are also distinguishable by their frequency ranges. Low-frequency, e.g., 30 KHz to 500 KHz, systems 100 have short reading ranges and lower system costs. They are commonly used in security access, asset tracking, and animal identification applications. High-frequency, e.g., 850 MHz to 950 MHz and 2.4 GHz to 2.5 GHz 100 systems offer long read ranges, e.g., greater than 90 feet, high reading speeds, and are used for such applications as railroad car tracking and automated toll collection; however, the higher frequency RFID systems 100 typically result in higher system costs.
RFID systems employ a type of modulation known as backscatter modulation. In a backscatter system, the tags do not generate their own RF carrier signal. Rather, interrogator 102 generates a carrier signal that it broadcasts within its coverage area. The tags then reflect this signal back to interrogator 102. The reflection of the carrier signal is termed backscatter. In order to communicate information on the backscattered signal, the tag will alternatively reflect and not reflect the signal in order to indicate “1”s and “0”s to interrogator 102. This is termed backscatter modulation.
The significant advantage of all types of RFID systems 100 is the noncontact, non-line-of-sight nature of the technology. Tags 106 can be read through a variety of substances such as snow, fog, ice, paint, crusted grime, and other visually and environmentally challenging conditions, where barcodes or other optically read technologies cannot typically be used. RFID tags 106 can also be read in challenging circumstances at high speeds, often responding in less than 100 milliseconds. RFID has become indispensable for a wide range of automated data collection and identification applications that would not be possible otherwise.
One area where passive RFID tags have proven potentially useful is in tracking documents. For example, a passive RFID tag shaped like a small sticker, or label can be affixed to a highly confidential document in order to track access to and the location of the document. In other words, if someone wants to access the document, which can be stored in a cabinet, then they can be required to scan the document out. Likewise, they can be required to scan the document in once they have returned it. In this manner, a record of when the document is “checked-out” can be maintained. Such a process can be combined with security mechanism limiting access to the cabinet as well as a mechanism to log who checked the document out.
Such a system, however, can prove cumbersome if there a many documents being checked in an out by multiple users. For example, in a conventional system, each document must be scanned in and out individually, otherwise the tags on each document will interfere with each other. RFID tags suffer from detuning and cross coupling (sharing energy) when two or more tags are placed in each other's effective area. The effective area is ¼ of the Radio Frequency (RF) wave length. For example, at 2401 MHz this is equivalent to 1.2 inches. When a document is tagged and then stacked on top of another tagged document, the result are two tag documents where the RFID tag is separated only by the thickness of the sheet of paper. This separation is insufficient to allow the two tags to work optimally resulting in the inability of the interrogator to communicate properly to the RFID tag. This problem is compounded when more tag documents are stacked.